(1) Field of the Invention
The present invention relates to a monitor system for monitoring the starting of a rotary wing aircraft. The invention also relates to a rotary wing aircraft fitted with such a system and to a method of starting a turboshaft engine that make use of the monitor system.
The invention thus lies in the field of devices for monitoring the power plant of rotary wing aircraft.
(2) Description of Related Art
Conventionally, a rotary wing aircraft has a power plant provided with a rotor that provides at least some of the lift of the aircraft. The rotor may also contribute to providing at least some of the propulsion of the aircraft. Thus, a helicopter has a main rotor that provides it both with lift and with propulsion.
The rotor is driven in rotation by a main gearbox that is driven by at least one engine.
For example, the power plant includes a turboshaft engine having a gas generator and a free turbine for delivering power. The free turbine is then linked to the main gearbox by a drive train for transmitting power.
The drive train for transmitting power is usually fitted with an overrunning clutch or “freewheel”. The freewheel has a driving portion that is driven in rotation by an engine, and a driven portion that is connected to the main gearbox.
Such a freewheel has the function of enabling the rotor to be driven by the engine while, in contrast, preventing the engine being driven by the drive train. The freewheel may be advantageous in particular during a stage of autorotation of the rotary wing, for example.
Such a freewheel can become degraded while it is in use. Under such circumstances, a degraded freewheel can slip, constituting a location where mechanical slip takes place between two components.
For example, the freewheel may have rollers interposed between the driving portion and the driven portion. The rollers then allow the driven portion to be driven by the driving portion. Nevertheless, the rollers may become eroded and they might no longer perform their function correctly.
When starting the engine, a worn freewheel can lead to mechanical rupture of the power drive train. If the driving portion of the freewheel no longer drives the driven portion, then the engine runs the risk of having its free turbine in an overspeed situation. If mechanical transmission between the driving portion and the driven portion is erratic, then the engine can be subjected to mechanical jolting in the event of the driving and driven portions suddenly re-engaging.
Slip in the freewheel can also lead to abnormal wear of mechanical elements present between the engine and the rotor of the rotary wing, e.g. as a result of repeated jolting.
Consequently, when starting the engine(s) of an aircraft, a pilot tends to perform visual verification that the rotor is being driven in rotation. If the rotor is set into rotation, then the pilot can deduce that the drive train is operating correctly.
The flight manual of an aircraft may require the pilot to verify visually that the rotor is rotating as from a threshold speed of rotation of the engine.
If the rotor is not set into rotation, the pilot then stops the engine.
The remote technological background in the field of the invention includes the following documents: U.S. Pat. No. 4,231,092 A, U.S. Pat. No. 5,799,748 A, and JP 2007 270 770 A.
Document U.S. Pat. No. 4,231,092 A proposes to replace an automatic transmission system having a clutch. A freewheel effect is simulated by a microprocessor. Slip is detected when the torque upstream from the clutch is negative.
Document U.S. Pat. No. 5,799,748 A describes means for detecting the current state of a freewheel system. That document does not describe the specific circumstance of the freewheel slipping and it is directly entirely to the remote automobile field.
Document JP 2007 270 770 A suggests measuring the speed of rotation of the rotor in order to detect slip of a freewheel.
Also known are the following documents: U.S. Pat. No. 5,046,923, GB 2 193 535, and US 2005/278084.